I had a hernia operation just over 1 week ago using open surgery and mesh, the hernia was quite large and had been in my scrotum for some time. Everything feels fine with it, the pain has gone and most of the swelling. The only problem is that the epididymis on the hernia side is swollen, not tender, just larger than it was before. Is this normal, will it go away or should I see my GP.

Yes, you should see your GP or a urologist or go back to the surgeon.
I had an open hernia op in the UK in 1995 and gradually developed swelling of the testicle on that side (right) up to the point where it was about four times the size of the other one. It never gave me any pain but I probably should have had it treated earlier. It was eventually operated on here in Belgium in May 2007 and now seems fine.